Polarizable photoactive and electroactive polymers and light emitting devices

ABSTRACT

The invention relates to poly(phenylene vinylene) polymers substituted with dendritic sidechains to enhance main-chain separation in the solid state. The polymers are synthesized by the Heck polymerization and have a weight-average molecular weight of 20,000 to 60,000 Daltons. The polymers are self-ordering in the solid state and have thermotropic liquid crystalline phases. The polymers show enhanced photoconductivity, better charge transport capability and improved polarized light emission.

BACKGROUND OF THE INVENTION

[0001] 1. FIELD OF THE INVENTION

[0002] The present invention relates to electroluminescent andphotoluminescent polymer compositions and processes for theirpreparation and use in, for example, electroluminescent devices such aselectroluminescent displays and liquid crystal displays.

[0003] 2. DESCRIPTION OF THE RELATED ART

[0004] Electroluminescent (EL) devices are structures which emit fightwhen subject to an applied electric field. The usual model for thephysical process in a semiconductor used in this way is through theradiative combination of electron-hole pairs which are injected into thesemiconductor from opposite electrodes. Common examples arelight-emitting diodes based on Gap and similar III-V semiconductors.Though these devices are efficient and widely used, they are limited insize, and are not easily or economically used in large area displays.Alternative materials which can be prepared over larger areas are alsoknown. Among the inorganic semiconductors, most effort has been directedto ZnS, which has considerable practical drawbacks and primarily poorreliability. The mechanism in ZnS is believed to be one in whichacceleration of one type of carrier through the semiconductor under astrong electric field causes local excitation of the semiconductor whichrelaxes through radiative emission.

[0005] Among organic materials, simple aromatic molecules such asanthracene, perylene and coronene are known to showelectrolumninescence. The practical difficulty with these materials is,as with ZnS, their poor reliability, together with difficulties indeposition of the organic layers and the current-injecting electrodelayers. Techniques such as sublimation of the organic material sufferfrom the disadvantage that the resultant layer is soft, prone torecrystallization, and unable to support high temperature deposition oftop-contact layers. Techniques such as Langmuir-Blodgett film depositionof suitably-modified aromatics suffer from poor film quality, dilutionof the active material, and high cost of fabrication.

[0006] Solid-state light-emitting diodes (LEDs) have found widespreadapplication in displays, as well as in a variety of other applications.LEDs are typically fabricated from conventional semiconductors, forexample, gallium arsenide (GaAs), typically doped with aluminum, indium,or phosphorus. Using this technology, however, it is very difficult tomake large area displays. In addition, the LEDs made of these materialsare typically limited to the emission of light at the long wavelengthend of the visible spectrum. For these reasons, there has beenconsiderable interest for many years in the development of suitableorganic materials for use as the active (light-emitting) components ofLEDs. The utilization of semiconducting organic polymers (i.e.,conjugated polymers) in the fabrication of LEDs expands the use oforganic materials in electroluminescent devices with the possibility ofsignificant advantages over existing LED technology.

[0007] Among the most recent discoveries was the discovery thatconjugated polymers are particularly well suited for this purpose inthat they provide excellent charge transport characteristics and usefulquantum efficiencies for luminescence. Conjugated polymers are animportant class of light emitting polymers for electroluminescent (EL)devices. A conjugated polymer is a polymer which possesses a delocalisedπ-electron system along the polymer backbone; the delocalised π-electronsystem confers semiconducting properties to the polymer and gives it theability to support positive and negative charge carriers with highmobilities along the polymer chain. Such polymers are discussed, forexample, by K H. Friend in Journal of Molecular Electronics 4 (1988)January-March, No. 1, pages 37 to 46. The most popular of the materialssuitable for this use is poly (phenylene vinylene) (PPV) which iscapable of being prepared in the form of a high quality film whichevidences strong photoluminescence in a band centered near 2.2 eV.

[0008] There are two principal approaches to the fabrication ofconjugated polymer thin films, namely, the precursor approach and theside chain approach. The former relies on the preparation of a solubleprecursor polymer which can be cast into thin films. The precursorpolymer can then be converted to the final conjugated polymer filmsthrough solid-state thermo- or photo-conversion. Friend et al., refersto EL devices based on poly(p-phenylene vinylene) (PPV) thin filmsderived from a sulfonium precursor route, see, e.g., U.S. Pat. No.5,247,190, issued Sep. 21, 1983, to Friend et al. Friend et al. alsorefers to an electroluminescent device having a semiconductor layer inthe form of a thin dense polymer film including at least one conjugatedpolymer, a first contact layer in contact with a first surface of thesemiconductor layer, and a second contact layer in contact with a secondsurface of the semiconductor layer. The polymer film of thesemiconductor layer has a sufficiently low concentration of extrinsiccharge carriers that, on applying an electric field between the firstand second contact layers across the semiconductor layer so as to renderthe second contact layer positive relative to the first contact layer,charge carriers are injected into the semiconductor layer and radiationis emitted from the semiconductor layer. The polymer film can be apoly(p-phenylene vinylene) wherein the phenylene ring may optionallycarry one or more substituents each independently selected from alkyl,alkoxy, halogen or nitro.

[0009] What is needed is a dendritic polymer, specifically apoly(phenylene vinylene) polymer for use in an electroluminescentdevice, which polymer is self-ordering in the solid state, hasthermotropic liquid crystalline phases, enhanced photoconductivity,better charge transport capability, and improved polarized lightemission over prior poly(phenylene vinylene) polymers.

SUMMARY OF THE INVENTION

[0010] The present invention relates to dendritic polymer such as apoly(phenylene vinylene) polymer having dendritic sidechains. Thedendritic sidechains enhance the main-chain poly(phenylene vinylene)separations in the solid state. The poly(phenylene vinylene) polymersaccording to the present invention may be synthesized using, e.g., theHeck polymerization method to achieve a resultant weight-averagemolecular weight of from about 20,000 to about 60,000 Daltons. Thepolymers are self-ordering in the solid state, have thermotropic andlyotropic liquid crystalline phases and show enhanced photoluminescenceefficiency and polarized light emission over prior poly(phenylenevinylene) polymers.

[0011] The present invention also provides processes for preparingsoluble poly(phenylene vinylene) polymers having dendritic side chains,which polymers are self-ordering in the solid state, have thermotropicliquid crystalline phases, enhanced photoconductivity, better chargetransport capability, and improved polarized light emission.

[0012] The above and other advantages and features of the invention willbe more clearly understood from the following detailed descriptiontogether with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 shows a reaction scheme for a polymer according to a firstembodiment of the present invention.

[0014]FIG. 2 shows a reaction scheme for a polymer according to a secondembodiment of the invention.

[0015]FIG. 3 shows the UV spectra for the polymers of the first andsecond embodiments of the invention in tetrahydrofuran solutions and inthin films.

[0016]FIG. 4 shows X-ray diffraction patterns for the polymer films ofthe first and second embodiments of the invention prepared fromdifferent solvents.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0017] Reference will now be made in detail to the preferred embodimentsof the invention, which, together with the drawings and the followingexamples, serve to explain the principles of the invention. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatother embodiments may be utilized, and that structural and chemicalchanges may be made without departing from the spirit and scope of thepresent invention.

[0018] An electroluminescent material is a material which emits lightupon application of an electric field. A photoluminescent material is amaterial which emits light upon excitation by light. Polarizedelectroluminescence and photoluminescence occur when the material hasmacroscopic ordering (molecules are aligned in a direction) so as togenerate polarized light. Polarized light sources are useful in thefabrication of brighter liquid crystal displays.

[0019] Liquid crystallinity is a state between crystalline solid phaseand isotropic liquid phase. In this state, the molecules are notpositionally ordered as in a crystal, but they spontaneously align withneighboring molecules, giving long-range orientational order. Thislong-range orientational order facilitates macroscopically aligning thematerial by various means.

[0020] A material may be ordered macroscopically in many ways. Forexample, if a material is a liquid crystalline material, it can bealigned in the liquid crystalline state by shear force, electric field,or magnetic field. If a material is not liquid crystalline, it can bealigned by mechanical stretching, blending with another material,dissolving in a solvent and spread to form a monolayer film, or by selfassembly. In order for a material to be ordered by mechanicalstretching, the material must have high mechanical strength. Otherwise,the material tends to break and the thickness of the material isdifficult to maintain at a uniform thickness. Also, alignment bymechanical stretching requires that the material be cooled quickly sothat it does not relax from the aligned state.

[0021] Macroscopic ordering may also be achieved by mixing the materialwith another stretchable polymer, such as polyethylene. However, such ablend generally cannot be used for electroluminescent devices since thematrix polymer is typically an insulator. A third way to achievemacroscopic ordering is by dissolving the material in a solvent andspreading to form a monolayer film, also known as the Langmuir-Blodgetttechnique. The material can be dissolved in an organic solvent and thesolution can be spread on a surface to form a monolayer film. This filmcan then be compressed by a blade to align molecules in the film.Eventually, the aligned film can be transferred to a substrate. Thedrawback of this method is that it requires the material to beamphiphilic. In addition, the technique itself is tedious and only asmall area of film can be made.

[0022] Materials can also be ordered by forming self-assemblingmaterials which self order themselves and form a macroscopically orderedfilm. For this method special molecules have to be designed andsynthesized.

[0023] The primary factor in determining whether a substance exhibitsliquid crystallinity is molecular shape. For example, rod-like moleculesare commonly found to exhibit liquid crystal phases. A seminal theoryfor the formation of liquid crystal nematic phases was developed by LarsOnsager in his paper “The Effects of Shape on the Interaction ofColloidal Particles” in the Annals of the New York Academy of Sciences,vol. 51, Art. 4, pp. 627-659 (1949). Onsager considered rod-likemolecules in a solution with no thermal interactions, only stericrepulsions between the rods. The rods have a diameter D and length L.Onsager predicted a liquid crystalline phase when the aspect ratio L/Dis sufficiently large. Based on a summary of Onssager's theory given inthe book “The Theory of Polymer Dynamics” by M. Doi and S. F. Edwards(Oxford: Belfast) 1986, the ratio L/D should-be over about four in asolution of pure rod-like molecules. In the polymers according to thepresent invention, the molecular architecture is more like a rigid rodwith regularly attached bulky spheroids on the sides. Including thespheroids in an estimate for the effective diameter and taking intoaccount the average molecular weight for the polymers synthesizedaccording to the present invention, the polymers have an L/D of about 3.Thus, it would not be expected based upon Onsager's theory that thepolymers of the present invention would exhibit liquid crystallinity.

[0024] Rigid-rod conjugated polymers with long alkyl chains can alsohave liquid crystallinities. The advantage of rigid-rod conjugatedpolymers with dendritic side chains over rigid-rod conjugated polymerswith long alkyl chains is that the melted liquid of conjugated polymerswith dendritic side chains tend to be less viscous. Since the viscosityof a polymer is usually a function of its molecular weight, thesidechains of the conjugated polymer can act as a solvent or lubricantand thus reduce the viscosity of the polymer. With the same backbonemolecular weight, the more sidechains the polymer has, the less viscousit is, and the easier for it to be aligned in the liquid crystallinestate. Lyotropic liquid crystalline state is found with a solution of asoluble molecule at a certain temperature and concentration range.Usually, high solution concentration is needed to form lyotropicliquidcrystals. Rigid-rod conjugated polymers with linear alkyl chains do nothave sufficient solubility to form lyotropic liquid cystalline polymers.However, with dendritic sidechains, these polymers are soluble inorganic solvents and lyotropic liquid crystalline phases were found.These types of poylmers may also be sheared in the lyotropic liquidcrystalline phases from a solution.

[0025] Also, more functional groups can be attached as sidechains sincedendrimers have many branches for functionalizations. This property isuseful for photoluminescence. For example, if the polymer has anarchitecture having a rod backbone with several dendritic chains, thedendritic chain elements can absorb light at a first wavelength andtransfer the energy to the backbone which may then emit light at alonger wavelength. If the concentration of the backbone is low (lessthan 5%), the optical loss due to absorption of the dendritic chainswill be low since the second wavelength is shifted away from the strongabsorption edge of the dendritic sidechains. See, M. Berggren et al.,Light Amplification in Organic Films using Cascade Energy Transfer, 389Nature 466-69 (1997).

[0026] Poly(phenylene vinylene) polymers and derivatives have manyapplications as active materials in optoelectronic devices. PPV withoutany substituents is insoluble in organic solvents and is usuallyprocessed from its precursor polymer by a conversion reaction. Varioussidechains can be attached to the PPV backbone to render it soluble andprocessable. Commonly used sidechains are simple linear alkyl and alkoxysubstituents. These polymers are classified as “hairy rod” polymers andhave been found to be thermotropic liquid crystals. However, these typesof polymers tend to form aggregates and result in low photoluminescenceand electroluminescence quantum efficiencies due to aggregationquenching. To solve this problem, branched chains, such as2-ethylhexyloxy in poly(2-methoxy-5-(2-ethylhexyloxy)phenylene vinylene)(MEH-PPV), and bulky substituents have been used. Other approachesinvolve the formation of nano-sized PPV domains by self-assembly or useof liquid crystals as templates. In addition, dilution of PPVchromophores using block copolymers or polymer blends and deliberateinclusion of cis PPV are effective methods.

[0027] In the present invention, the synthesis and structue of branchedand hyperbranched (dendritic) sidechain substituted PPVs are used toenhance main-chain PPV separations in the solid state. The dendriticsubstituent according to the present invention may be an aromaticetheral moeity with one or a plurality of substituent having C₆-C₁₂alkyl groups. Interestingly, these polymers were found to exhibitthermotropic liquid crystalline order. Liquid crystallinity can be usedto form highly ordered photoluminescent films for generating polarizedlight. Specific examples of dendritic PPV's according to the presentinvention include:

[0028] The dendritic polymers of the present invention a repeating unitof the formula:

[0029] wherein A is selected from the group consisting of,

[0030] wherein B is selected from the following:

[0031] wherein X is a dendritic substituent or a methyl group;

[0032] wherein D is selected from:

[0033] and where R may be a dendritic substutuent, an alkyl substituentand/or an alkoxy substituent provided that at least one R is a dendriticsubstituent. Preferred dendritic substituents include R¹, R² and R³,wherein R¹ is a compound of the formula:

[0034] and R² is a compound of the formula:

[0035] and R³ is a compound of the formula:

[0036] where n is an integer from about 5 to about 25 and R⁴ is anyC₂-C₁₈ alkyl group. Preferably, n is an integer from about 10 to about20, most preferably about 15. R⁴ is independently selected dependingupon the desired physical characteristics of the molecule. R⁴ may be abranched or straight chain group. In particular, R⁴ is preferably aC₆-C₁₂ alkyl group, most preferably an n-C₆ or an n-C₁₂ alkyl group.

[0037] An exemplary synthesis of a dendritic sidechain PPV is shown withreference to FIGS. 1 and 2. Percec et al., 118 J. Am. Chem. Soc. 9855,1996 describes a basic method for fabrication of PPV's and is hereinincorporated by reference.

[0038] Reference is now made to FIG. 1. Methyl-3,4,5-trihydroxybenzoatewas chosen as the core for building up the dendritic sidechains sincehighly branched molecules can be formed frommethyl-3,4,5-trihydroxybenzoate even at low yield.

[0039] As shown with reference to FIG. 1, themethyl-3,4,5-trihydroxybenzoate is reacted with n-hexyl bromide andpotassium carbonate in the presence of dimethyl formamide (DMF) at 65°C. The reaction results in a methyl-3,4,5-trihexyl ether benzoateintermediate product as shown below.

[0040] The intermediate methyl-3,4,5-trihexyl ether benzoate is thenreacted with etheral lithium aluminum hydrate at a temperature of about0° C. followed by thionyl chloride and dichloromethane at a temperatureof about 25° C. to form a 3,4,5-trihexyl-methylbenznyl chlorideintermediate as shown below.

[0041] The intermediate ,4,5-trihexyl-methylbenznyl chloride is thenreacted with 2,5-diiodo hydroquinone in the presence of potassiumcarbonate and DMF at 65° C. to form monomerl.

[0042] where R¹ is

[0043] The palladium-catalyzed Heck reactions are used for thepolymerizations. See, Heck, R. F. Org. React. 1982. More specifically,monomerl is then reacted with divinyl benzene in the presence ofpalladium diacetate, POT, triethyl nitrate and DMF at a temperature ofabout 100° C. to form polymerl (PPVD1)as shown below

[0044] where n is from about 5 to about 25, preferably about 15 andwhere R¹ is of the formula set forth above. The Heck reaction waspreviously found to have side-reactions in which both the 1 and2-position of the vinyl group can react with phenyl halides with muchlower reactivity at the 2-position. In the present invention, due tothis side-reaction the vinyl groups were not visible in ¹H NMRSubsequent observations of self-ordering and liquid crystallineproperties of this polymer indicate that few structural defects exist.

[0045] Reference is now made to FIG. 2. A polymer according to a secondembodiment of the invention is formulated according to the reactionscheme set forth in FIG. 2. Again, methyl-3,4,5-trihydroxybenzoate waschosen as the core for building up the dendritic sidechains since highlybranched molecules can be formed from methyl-3,4,5-trihydroxybenzoateeven at low yield.

[0046] The methyl-3,4,5-trihydroxybenzoate is reacted with n-hexylbromide and potassium carbonate in the presence of dimethyl formamide(DMA) at 65° C. The reaction results in a methyl-3,4,5-trihexyl etherbenzoate intermediate product as shown below.

[0047] The intermediate methyl-3,4,5-trihexyl ether benzoate is thenreacted with etheral lithium aluminum hydrate at a temperature of about0° C. followed by thionyl chloride and dichloromethane at a temperatureof about 25° C. to form a 3,4,5-trilheql-methylbenznyl chlorideintermediate as shown below.

[0048] The intermediate ,4,5-trihexyl-methylbenznyl chloride is thenreacted with methyl-3,4,5-trihydroxybenzoate in the presence ofpotassium carbonate and DMF at 65° C. as shown below to form thecompound depicted below.

[0049] where R¹ is shown below

[0050] The above compound is then reacted with etherat lithium aluminumhydrate at a temperature of about 0° C. followed by thionyl chloride,dichloromethane, and 2,5-di-tert-butylpyridine at a temperature of about25° C. to form an intermediate compound as shown below.

[0051] The above compound is reacted with 2,5-diiodo hydroquinone in thepresence of potassium carbonate and DMF at 65° C. to form monomer2 whereR² has the formula shown below.

[0052] where R² is

[0053] Palladium-catalyzed Heck reactions are again used for thepolymerizations. See, Heck, R. F. Org. React. 1982. Monomer2 is thenreacted with divinyl benzene in the presence of palladium diacetate,POT, triethyl nitrate and DMF at a temperature of about 100° C. to formpolymer2 (PPVD2) as shown below

[0054] where n is from about 5 to about 25, preferably about 15 andwhere R² is of the formula set forth above. As discussed above, the Heckreaction was previously found to have side-reactions where both the 1and 2-position of the vinyl group can react with phenyl halides withmuch lower reactivity at the 2-position. In the present invention, dueto this side-reaction the vinyl groups were not visible in ¹H NMRSubsequent observations of self-ordering and liquid crystallineproperties of this polymer indicate that few structural defects exist.

[0055] The chemical structures-and syntheses of the polymers are alsoshown in FIGS. 1 and 2. An elemental analysis found the followingresults for monomers 1 and 2 and polymers PPVD1 and PPVD2: CompoundMolecular formula Melting Point Molecular Weight Monomer 1 C₅₆H₈₈I₂O₈63-66° C. Monomer 2 C₁₇₀H₂₆₈O₂₆I₂ 73-76° C. PPVD1 25,300 Daltons PPVD262,800 Daltons

[0056] An elemental analysis of PPVDI found C, 77.28%; H, 9.36%. Anelemental analysis for PPVD2 found C, 75.49%, H, 9.57%. Physical testsfurther indicated that PPVD1 had a polydispersity of 2.31 while PPVD2had a polydispersity of 1.55. These two polymers according to thepresent invention, namely PPVD1 and PPVD2 as shown in FIGS. 1 and 2, aresoluble in common organic solvents such as tetrahydrofuran (THF),chloroform, and toluene. The relative molecular weights of the polymerswere determined by Gel Permeation Chromatography (GPC) using THF aseluent and calibrated with polystyrene standards at 35° C. Both PPVD1and PPVD2 have molecular weights greater than 10,000 Dalton. Theabsolute molecular weight was also determined for PPVD2 with laser lightscattering measurements and was found to be about two times higher thanits relative molecular weight as determined by GPC. Thus the polymersaccording to the present invention have a weight average molecularweight in the range of from about 10,000 to greater than 60,000. Thenumber of repeating units, n, calculated from absolute number-average MWis thus from about 10 to about 25, and most preferably about 15.

[0057] Reference is now made to FIG. 3. The UV-absorption spectra weremeasured for both solutions (in THF) and thin films. PPVD1 and PPVD2have similar absorption spectra in solution characterized by maximumabsorption (λmax) at around 215 nm, resulting from π-π* transition (thedifference between the r electrons in a ground state, π, and the πelectrons in an excited state, π* ) of isolated phenyl rings from thesidechains, and at 460 nm due to the mar* transition of the PPVbackbone. The spectrum of a PPVD2 film prepared from evaporation of theTHF solvent has the same λmax as its solution spectrum but with aslightly broadened absorption linewidth. While not wishing to be boundby theory, it is believed that the fact that λmax remains unshiftedindicates weak interactions between the conjugated backbones in thesolid state due to the bulky dendritic sidechains. As compared to PPVD2,the spectrum of the PPVD1 film is red-shifted in THF solution, which iscommonly observed for alkyl-substituted PPVs resulting from π-π*stacking of the conjugated backbones.

[0058] Reference is now made to FIG. 4. X-ray diffraction measurementswere performed on films prepared from different solvents. Both polymerswere found to self-order with polymer backbones parallel to thesubstrate and with a strong solvent dependence on the degree ofordering. For a PPVD1 film cast from chloroform at room temperature, avery sharp diffraction peak was found at 2θ=2.32°(d=38 Å), where 20 isthe angle where the x-ray diffracted pattern signal is detected asdetermined by Brag's equation. Higher orders were also visibleindicating a very high degree of ordering under this condition (FIG.4A). However, when toluene was used as the solvent and evaporated atabout 50° C., a diffuise peak was found at 2θ=3.35°−3.55° correspondingto a spacing of 25-26 Å (FIG. 4B). A much narrower and intensediffraction peak at 2θ=3.55° (d=25 Å) was found for a film cast from1,1,2,2-tetrachloroethane at 50° C. (FIG. 4C). While not wishing to bebound by theory, it is believed that the 25 Å and 38 Å spacingsrepresent the backbone separations for two alternative forms ofinterdigitation. The 38 Å distance would thus be very reasonable forextended sidechains essentially transverse to the PPV backbone withlittle or no interdigitation.

[0059] For PPVD2, similar spacings of about 22 to 26 Å were observed forall films prepared from different solvents except that the diffractionpeak appeared to broaden for the film prepared from chloroform solution(FIG. 4D) while it was narrower from toluene (FIG. 4E) and1,1,2,2-tetrachloroethane (FIG. 4F). This spacing correspondsapproximately to the radius of the dendritic sidechain; this resulttherefore indicates that the sidechains of PPVD2 are interdigitated inthe solid state. The X-ray diffraction pattern for adiheptoxy-substituted poly (phenylene vinylene) prepared using the samepolymerization reaction was also obtained for comparison. However, onlydiffuse peaks were seen for the film cast from chloroform solution (FIG.4G). Therefore, the bulky dendritic sidechains appear to be much moreeffective in promoting self-ordering in the PPV polymer according to thepresent invention.

[0060] Notably, both polymers according to the present invention, PPVD1and PPVD2, were found to exhibit thermotropic nematic liquid crystalphases. Thin films cast from THF were studied by optical microscopy.Brightness arises from the birefringence of the liquid crystal state,and the numerous thread-like dark lines are typical of the schlierentexture of nematics. To verify that the nematic phase is not related tothe solvent used to cast the films, another set of films was spin coatedon glass. In the polymers according to the present invention, thesolvent evaporates so quickly that the films appear amorphous at roomtemperature. However, upon heating the films to around 180° C. thencooling, schlieren texture spontaneously forms. This shows that thenematic state is thermodynamically preferred in the absence of solvent.The nematic-to-isotropic transition temperature (T_(NI)) was measured onmuch thicker specimens (≈10-15 μm) to avoid finite thickness effects andto give a strong birefringence signal. Upon heating, the birefringencefades and the field eventually becomes dark between crossed polarizers.Upon cooling, the birefringence returns. From these observations, thenematic-to-isotropic transition temperatures for the polymers accordingto the present invention were determined to be about 200±5° C. for PPVD1and about 211±5° C. for PPVD2. The nematic character of PPV derivativescan thus be exploited in a number of ways to generate strongly alignedfilms, resulting in enhanced photoconductivity and better chargetransport capability, as well as polarized light emission. For example,the polymers according to the present invention can be strongly alignedby deformation. After a thin film of either material is sheared, itexhibits a persistent birefringence with few defects. This reflects ahigh degree of polymer chain alignment.

[0061] Among conjugated polymers, liquid crystallinity is common for the“hairy rod” polymers, which have rigid backbones and flexiblesidechains. The dendritic sidechain polymers according to the presentinvention are also internally flexible. However, compared to linearalkyl chains, they are much bulkier and the resulting interchainpackings are expected to be more sensitive to their spatial shapes.

[0062] In another embodiment of the present invention, conjugateddendritic PPV polymer-based electroluminescent devices are prepared on asubstrate. The substrate is preferably transparent and nonconducting.The substrate can be a rigid material such as a rigid plastic includingrigid acrylates, carbonates, and the like, or rigid inorganic oxidessuch as glass, quartz, sapphire, and the like. The substrate can also bea flexible transparent organic polymer such as polyester—for examplepolyethyleneterephthalate, flexible polycarbonate, poly (methylmethacrylate), poly(styrene) and the like. The thickness of thissubstrate is not critical. In the device of the present invention, theconjugated polymer is a poly (phenylene vinylene) [PPV] having dendriticsidechains as set forth herein. A first charge injecting contact layeris a thin layer of aluminum, one surface having a thin oxide layer. Thefirst surface of the semiconducting layer is in contact with the oxidelayer. The second charge injecting contact layer is a thin layer ofaluminum or gold to form an electroluminescent device.

[0063] The present invention relates to new poly(phenylene vinylene)substituted with dendritic sidechains. These polymers are self-orderingpolymers in the solid state and yield thermotropic nematic phases. It isto be understood, however, that the invention has broader applicabilityand includes electroluminescent devices including mixtures of thepolymers of the present invention with known electroluminescentpolymers. Similarly, the process described above is but one method ofmany that could be used to fabricate the polymers according to thepresent invention.

[0064] Thus, the present invention provides a new poly (phenylenevinylene) having dendritic sidechains and a method for preparing thesame. It should again be noted that although the invention has beendescribed with specific reference to particular poly (phenylenevinylene) polymers and particular dendritic sidechains, the inventionhas broader applicability and may include electroluminescent devicesincluding mixtures of the polymers of the present invention and otherpolymers in any electroluminescent device, such as a light emittingdiode, or the like. Accordingly, the above description and accompanyingdrawings are only illustrative of preferred embodiments which canachieve the features and advantages of the present invention. It is notintended that the invention be limited to the embodiments shown anddescribed in detail herein. The invention is only limited by the scopeof the following claims.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A compound having a repeating unit of theformula:

wherein A is selected from the group consisting of,

wherein B is selected from the following:

wherein X is a dendritic substituent or a methyl group; wherein D isselected from:

wherein R comprises a dendritic substituent, an alkoxy substituent or aan alkyl substituent, provided that at least one R group is a dendriticsubstituent; and n is an integer from about 5 to about
 25. 2. Thecompound according to claim 1, wherein R comprises at least onesubstituent selected from the group consisting of OR¹, OR², OR³ and amethoxy group, wherein R¹ is a substituent of the formula:

and R² is a substiuent of the formula:

and R³ is a substituient of the formula:

where n is an integer from about 5 to about 25 and R⁴ is independentlyselected from a C₂-C₁₈ alkyl group.
 3. The compound according to claim2, wherein n is from about 10 to about
 20. 4. The compound according toclaim 2, wherein n is about
 15. 5. The compound according to claim 2,wherein the compound has a molecular weight greater than 10,000 Daltons.6. The compound according to claim 2, wherein the compound has amolecular weight of from about 20,000 to about 60,000 Daltons.
 7. Thecompound according to claim 2, wherein said compound is symmetrical. 8.The compound according to claim 2, wherein said compound isasymmetrical.
 9. The compound according to claim 2, wherein R comprisesfrom about 10 to about 90% by weight R¹ and from about 10 to about 90%by weight R².
 10. The compound according to claim 9, wherein R⁴ is aC₆-C₁₂ alkyl group.
 11. The compound according to claim 10, wherein R⁴is a C₆ alkyl group.
 12. The compound according to claim 10, wherein R⁴is a C₁₂ alkyl group.
 13. The compound according to claim 2, wherein Rcomprises from about 10 to about 90% by weight R¹ and from about 10 toabout 90% by weight R³.
 14. The compound according to claim 13, whereinR⁴ is a C₆-C₁₂ alkyl group.
 15. The compound according to claim 14,wherein R⁴ is a C₆ alkyl group.
 16. The compound according to claim 14,wherein R⁴ is a C₁₂ alkyl group.
 17. The compound according to claim 2,wherein R comprises from about 10 to about 90% by weight le and fromabout 10 to about 90% by weight R².
 18. The compound according to claim17, wherein R⁴ is a C₆-C₁₂ alkyl group.
 19. The compound according toclaim 18, wherein R⁴ is a C₆ alkyl group.
 20. The compound according toclaim 18, wherein R⁴ is a C₁₂ alkyl group.
 21. A compound having arepeating unit of the formula:

where R comprises a dendritic substituent or a methyl group, providedthat at least one R group is a dendritic substituent.
 22. The compoundaccording to claim 21, wherein R comprises at least one substituentselected from the group consisting of R¹, R², R³ and methyl, wherein R¹is a substituent of the formula:

and R² is a substiuent of the formula:

and R³ is a substituent of the formula:

where n is an integer from about 5 to about 25 and R⁴ is independentlyselected from a C₂-C₁₈ alkyl group.
 23. The compound according to claim22, wherein n is from about 10 to about
 20. 24. The compound accordingto claim 22, wherein n is about
 15. 25. The compound according to claim22, wherein the compound has a molecular weight greater than 10,000Daltons.
 26. The compound according to claim 22, wherein the compoundhas a molecular weight of from about 20,000 to about 60,000 Daltons. 27.The compound according to claim 22, wherein said compound issymmetrical.
 28. The compound according to claim 22, wherein saidcompound is asymmetrical.
 29. The compound according to claim 22,wherein R comprises from about 10 to about 90% by weight R¹ and fromabout 10 to about 90% by weight R².
 30. The compound according to claim29, wherein R⁴ is a C₆-C₁₂ alkyl group.
 31. The compound according toclaim 30, wherein R⁴ is a C₆ alkyl group.
 32. The compound according toclaim 30, wherein R⁴ is a C₁₂ alkyl group.
 33. The compound according toclaim 22, wherein R comprises from about 10 to about 90% by weight R¹and from about 10 to about 90% by weight R³.
 34. The compound accordingto claim 33, wherein R⁴ is a C₆-C₁₂ alkyl group.
 35. The compoundaccording to claim 34, wherein R⁴ is a C₆ alkyl group.
 36. The compoundaccording to claim 34, wherein R⁴ is a C₁₂ alkyl group.
 37. The compoundaccording to claim 22, wherein R comprises from about 10 to about 90% byweight R² and from about 10 to about 90% by weight R².
 38. The compoundaccording to claim 37, wherein R⁴ is a C₆-C₁₂ alkyl group.
 39. Thecompound according to claim 38, wherein R⁴ is a C₆ alkyl group.
 40. Thecompound according to claim 38, wherein R⁴ is a C₁₂ alkyl group.
 41. Acompound having a repeating unit of the formula:

where R comprises at least one substituent selected from the group

consisting of R¹, R², R³ and methyl, wherein R¹ is a substituent of theformula: and R² is a substituent of the formula:

and R³ is a substituent of the formula:

where n is an integer from about 5 to about 25, provided that at leastone R group is selected from R¹, R² or R³.
 42. The compound according toclaim 41, wherein n is from about 10 to about
 20. 43. The compoundaccording to claim 41, wherein n is about
 15. 44. The compound accordingto claim 41, wherein the compound has a molecular weight greater than10,000 Daltons.
 45. The compound according to claim 41, wherein thecompound has a molecular weight of from about 20,000 to about 60,000Daltons.
 46. The compound according to claim 41, wherein R comprisesfrom about 10 to about 90% by weight R¹.
 47. The compound according toclaim 41, wherein R comprises from about 10 to about 90% by weight R².48. The compound according to claim 41, wherein R comprises from about10 to about 90% by weight R³.
 49. A compound having a repeating unit ofthe formula:

where R comprises a substituent of the formula:

and where n is an integer from about 5 to about
 25. 50. The compoundaccording to claim 49, wherein n is from about 10 to about
 20. 51. Thecompound according to claim 49, wherein n is about
 15. 52. The compoundaccording to claim 49, wherein the compound has a molecular weightgreater than 10,000 Daltons.
 53. The compound according to claim 49,wherein the compound has a molecular weight of from about 20,000 toabout 60,000 Daltons.
 54. A compound having a repeating unit of theformula:

where R comprises a substituent of the formula:

and where n is an integer from about 5 to about
 25. 55. The compoundaccording to claim 54, wherein n is from about 10 to about
 20. 56. Thecompound according to claim 54, wherein n is about
 15. 57. The compoundaccording to claim 54, wherein the compound has a molecular weightgreater than 10,000 Daltons.
 58. The compound according to claim 54,wherein the compound has a molecular weight of from about 20,000 toabout 60,000 Daltons.
 59. A compound having a repeating unit of theformula:

where R comprises a substituent of the formula:

and where n is an integer from about 5 to about
 25. 60. The compoundaccording to claim 59, wherein n is from about 10 to about
 20. 61. Thecompound according to claim 59, wherein n is about
 15. 62. The compoundaccording to claim 59, wherein the compound has a molecular weightgreater than 10,000 Daltons.
 63. The compound according to claim 539,wherein the compound has a molecular weight of from about 20,000 toabout 60,000 Daltons.
 64. A method for forming a compound having arepeating unit of the formula:

said method comprising: reacting a monomer having the formula

with a divinyl benzene to form said compound, where R is independentlyselected from of the group consisting of R¹, R², R³ and methyl, providedthat at least one R group is R¹, R² or R3, wherein R¹ is

a substituent of the formula: and R² is a substituent of the formula:

and R³ is a substituent of the formula:

and where n is an integer from about 5 to about
 25. 65. The methodaccording to claim 64, wherein n is from about 10 to about
 20. 66. Themethod according to claim 64, wherein n is about
 15. 67. The compoundaccording to claim 64, wherein the compound has a molecular weightgreater than 10,000 Daltons.
 68. The compound according to claim 64,wherein the compound has a molecular weight of from about 20,000 toabout 60,000 Daltons.
 69. The method according to claim 64, where R isR¹.
 70. The method according to claim 69, wherein n is from about 10 toabout
 20. 71. The method according to claim 69, wherein n is about 15.72. The compound according to claim 69, wherein the compound has amolecular weight greater than 10,000 Daltons.
 73. The compound accordingto claim 69, wherein the compound has a molecular weight of from about20,000 to about 60,000 Daltons.
 74. The method according to claim 64,where R is R².
 75. The method according to claim 74, wherein n is fromabout 10 to about
 20. 76. The method according to claim 74, wherein n isabout
 15. 77. The method according to claim 74, wherein the compound hasa molecular weight greater than 10,000 Daltons.
 78. The method accordingto claim 74, wherein the compound has a molecular weight of from about20,000 to about 60,000 Daltons.
 79. The method according to claim 64,wherein said compound is formed by Paladium catalyzed polymerization.80. The method according to claim 64, wherein said compound is formed byPaladium catalyzed polymerization.
 81. The method according to claim 74,wherein said compound is formed by Paladium catalyzed polymerization.82. A method for forming a compound having a repeating unit of theformula:

said method comprising: reacting a monomer having the formula

a divinyl benzene to form said compound, where R is independentlyselected from of the group consisting of R¹, R², R³ and methyl, providedthat at least one R group is R¹, R² or R³, wherein W is a substituent ofthe formula:

and R² is a substituent of the formula:

and R³ is a substituent of the formula:

where n is an integer from about 5 to about 25 and R⁴ is independentlyselected from a C₂-C₁₈ alkyl group.
 83. The method according to claim82, wherein n is from about 10 to about
 20. 84. The method according toclaim 82, wherein n is about
 15. 85. The compound according to claim 82,wherein the compound has a molecular weight greater than 10,000 Daltons.86. The compound according to claim 82, wherein the compound has amolecular weight of from about 20,000 to about 60,000 Daltons.
 87. Themethod according to claim 82, wherein said monomer is reacted with saiddivinyl benzene to form a symmetrical polymer.
 88. The method accordingto claim 82, wherein said monomer is reacted with said divinyl benzeneto form an asymmetrical polymer.
 89. The method according to claim 82,where the monomer includes R¹ from about 10 to about 90% and includes R²from about 90 to about 10%.
 90. The method according to claim 82, wherethe monomer includes R¹ from about 10 to about 90% and includes R³ fromabout 90 to about 10%.
 91. The method according to claim 82, where themonomer includes R² from about 10 to about 90% and includes R³ fromabout 90 to about 10%.
 92. The compound according to claim 89, whereinR⁴ is a C₆-C₁₂ alkyl group.
 93. The compound according to claim 89,wherein R⁴ is a C₆ alkyl group.
 94. The compound according to claim 89,wherein R⁴ is a C₁₂ alkyl group.
 95. The compound according to claim 90,wherein R⁴ is a C₆-C₁₂ alkyl group.
 96. The compound according to claim90, wherein R⁴ is a C₆ alkyl group.
 97. The compound according to claim90, wherein R⁴ is a C₁₂ alkyl group.
 98. The compound according to claim91, wherein R⁴ is a C₆-C₁₂ alkyl group.
 99. The compound according toclaim 91, wherein R⁴ is a C₆ alkyl group.
 100. The compound according toclaim 91, wherein R⁴ is a C₁₂ alkyl group.
 101. A light emitting deviceincluding a conjugated polymer having a repeating unit of the formula:

where R comprises a substituent selected from the group consisting ofR¹, R², R³ and a methyl group, provided that at least one R group is R¹,R² or R³, wherein R¹ is a substituent of the formula:

R² is a substituent of the formula:

R³ is a substituent of the formula:

and where n is an integer from about 5 to about 25 and R⁴ isindependently selected from a C₂-C₁₈ alkyl group.
 102. The deviceaccording to claim 101, wherein n is from about 10 to about
 20. 103. Thedevice according to claim 101, wherein n is about
 15. 104. The deviceaccording to claim 101, wherein the conjugated polymer has a molecularweight greater than 10,000 Daltons.
 105. The device according to claim101, wherein the conjugated polymer has a molecular weight of from about20,000 to about 60,000 Daltons.
 106. The device according to claim 101,wherein R includes R¹.
 107. The device according to claim 101, wherein Rincludes R².
 108. The device according to claim 101, wherein R includesR³.
 109. The device according to claim 101, wherein said polymer issymmetrical.
 110. The device according to claim 101, wherein saidpolymer is asymmetrical.
 111. The device according to claim 101, whereinR⁴ is a C₆-C₁₂ alkyl group.
 112. The device according to claim 101,wherein R⁴ is a C₆ alkyl group.
 113. The device according to claim 101,wherein R⁴ is a C₁₂ alkyl group.
 114. The device according to claim 101,wherein said device is an electrolunimescent device.
 115. The deviceaccording to claim 101, wherein said device is a photoluminescentdevice.
 116. The device according to claim 101, wherein said device is alight emitting diode.